1. Field of the Invention
The invention relates generally to image projection systems, and particularly to three-dimensional projection systems.
2. Description of the Related Art
Video display and projection systems are ubiquitous devices with applications in many settings. Simple TV""s are a major tool in the communication of images for entertainment, news, and advertising. Computer monitors are an indispensable tool for displaying text, images, video, and other graphics. From the simplest black and white television to the most advanced computer-graphics monitors, however, video displays have hitherto been inherently two-dimensional devices. They display information on a flat surface, with any three-dimensional structure compressed into a single plane of view.
Efforts have been made on a number of fronts to present 3-D structures on 2-D media. One class of techniques involves stereo imaging, which uses the binocular aspect of the human visual system to simulate 3-D imagery. In these techniques, the viewer""s eyes are presented with two separate 2-D views of a 3-D object. Each eye is provided with the picture that would been seen from a point in space where that eye would be located while viewing the 3-D object. When the viewer""s eyes align to merge the separate images into a single image, parallax effects create the illusion of an image with depth.
3-D motion pictures are a common application of stereo imaging. In this application, two images are simultaneously projected onto a single movie screen using polarized light. The polarizations used in generating the two images are orthogonal to each other. When viewed directly, the screen shows a blurred combination of two separate images. When viewed through polarized xe2x80x9c3-D glasses,xe2x80x9d however, each of the viewer""s eyes sees only one of the two images. The viewer can then naturally combine the two images into a single fused 3-D image.
Stereo imaging is also used in xe2x80x9crandom-dot stereograms.xe2x80x9d These are pictures that initially appear to be a random collection of dots. When a viewer appropriately aligns his eyes, two separate collections of dots in the stereogram overlap, fusing into a single 3-D image.
Another technique used to present 3-D images is to combine several views of the same 3-D object into parallel strips on a single flat picture, and then to cover the picture with a lenticular screen so that each of the views is only visible when the picture is seen from a particular angle. While this technique does not rely on binocular vision, it is similar to stereo imaging in that the number of different views available to an observer is strongly limited. The observer can only view the 3-D object from those angles corresponding to the views that were combined into the single flat picture. The lenticular screen technique has been used in cameras that take 3-D photographs and in the production of xe2x80x9cholographicxe2x80x9d images on trading cards and other novelty items.
A simpler approach to displaying 3-D objects has been to display them as flat images with visual cues that suggest the solid nature of the objects. This technique generally starts with wire-grid drawings that illustrate the principal vertices, edges, and surfaces of a 3-D object. The illusion of depth is created by providing visual cues such as perspective: elements are drawn smaller when they are closer to a xe2x80x9cvanishing pointxe2x80x9d in the image. The objects may then be rendered into solid-looking structures by adding color and shading to the unhidden surfaces. Because of the small amount of information required to render a wire-grid drawing, this technique has found widespread use in computer animation. Nonetheless, it is inherently a 2-D technique: the position of the viewer""s eye does not affect what is seen by the viewer.
True holograms are interferometric recordings of three-dimensional objects onto a holographic recording medium. Unlike standard photographs, which record only the intensity of incident light, holograms record the phase as well as the intensity of the light that illuminates them. When appropriately illuminated by coherent light scattered from a three-dimensional object, a hologram records the wave fronts created by the object. During subsequent viewing, the hologram is appropriately illuminated by a light source. It functions as a diffraction grating, scattering the light in various directions. The hologram scatters the light in such a way as to recreate the originally recorded wavefronts. The result is a real or virtual image of the original three-dimensional object. Unlike the images generated by stereo images, lenticular stereographic images, wire-grid drawings, and other 2-D images, holograms provide a 3-D image that can be inspected from a number of different viewpoints selected by the observer.
Holograms tend to be largely monochromatic, since the interference effects used to record and display a hologram are highly wavelength-specific. They are also limited to the recording of static objects. Thus, holograms have not found widespread practical applications in video display technology.
Instead of recording a particular three-dimensional object, a hologram may be used as a diffraction grating that reproduces the effects of a particular optical element, such as a lens or a mirror. These xe2x80x9cholographic optical elementsxe2x80x9d (HOEs) may be far easier and less expensive to produce than their glass counterparts, especially when the optical element is complicated or must meet stringent tolerances. HOEs may generally be employed in any place where the corresponding glass optical element is used. HOEs have found applications in video projection systems, where large-dimension (wide) optical elements are required. They take the place of large lenses and other beam-shaping elements that can be expensive to produce in glass.
Described herein are systems and methods for projecting images that use switchable holographic optical elements (HOEs). These systems and methods can be used to project three-dimensional images or to project two-dimensional tiled images with increased size and/or resolution. One of the methods may include sequentially displaying first, second, and third color components of a first two-dimensional image at an object plane. The first two dimensional image represents a first slice of a three-dimensional image. As the first, second, and third color components are displayed, first, second and third HOEs may be activated so that the activated first switchable HOE focuses the first color component of the first two-dimensional image onto a first image plane, the activated second switchable HOE focuses the second color component of the first two-dimensional image onto the first image plane, and the wherein the activated third switchable HOE focuses the third color component of the first two-dimensional image onto the first image plane. It is noted that the first, second, and third HOEs may be activated concurrently or sequentially with the display of the first, second, or third color components of the first two-dimensional image. After the first, second, and third color components are displayed, the first, second and third switchable HOEs are deactivated. Then, first, second, and third color components of a second two-dimensional image are sequentially displayed. The second two-dimensional image represents a second slice of the three-dimensional image. A fourth switchable HOE may be activated to focus the first color component of the second two-dimensional image onto a second image plane. The second image plane is adjacent to the first image plane.
The switchable HOEs may be arranged in parallel on a common optic axis. Alternatively, they may be arranged side-by-side on a single plane. One embodiment of the projection system uses a combination of these arrangements: three sheets of HOEs are arranged in a stack, with each sheet capable of diffracting a single color of light. Each sheet includes an array of HOEs with varying focal lengths. The sheets are aligned in the stack so that each lens element for a given focal length on one sheet is coaxially aligned with lens element for the same focal length on the other two sheets.
The switchable HOEs can also be used in image projection method and system that projects an image composed of an array of xe2x80x9ctiles.xe2x80x9d Each tile is a single image displayed on a display device, such as a reflective LCD display. The display device is configured to sequentially display tile elements of the image. The image projection system also includes a switchable HOE configured to focus the two-dimensional display onto a sequence of positions on an image plane. The display device and the switchable HOE are synchronized so that tile elements are focussed onto their appropriate corresponding array positions in the image plane. The system sequences through the plurality of tile elements rapidly, displaying an entire image in less than 100 ms. Thus, the displayed image appears as a single, continuous image.